pkexec

Synopsis

DESCRIPTION

pkexec allows an authorized user to
execute PROGRAM as another
user. If username is not specified,
then the program will be executed as the administrative super
user, root.

RETURN VALUE

Upon successful completion, the return value is the return value
of PROGRAM. If the calling process is
not authorized or an authorization could not be obtained through
authentication or an error occured, pkexec
exits with a return value of 127. If the authorization could not
be obtained because the user dismissed the authentication
dialog, pkexec exits with a return value of
126.

AUTHENTICATION AGENT

pkexec, like any other PolicyKit application,
will use the authentication agent registered for the calling
process. However, if no authentication agent is available, then
pkexec will register its own textual
authentication agent. This behavior can be turned off by passing
the --disable-internal-agent option.

SECURITY NOTES

Executing a program as another user is a privileged
operation. By default the required authorization (See
the section called “REQUIRED AUTHORIZATIONS”) requires administrator
authentication. In addition, the authentication dialog presented
to the user will display the full path to the program to be
executed so the user is aware of what will happen:

The environment that PROGRAM will run
it, will be set to a minimal known and safe environment in order
to avoid injecting code
through LD_LIBRARY_PATH or similar
mechanisms. In addition the PKEXEC_UID
environment variable is set to the user id of the process
invoking pkexec. As a
result, pkexec will not allow you to run
X11 applications as another user since
the $DISPLAY and $XAUTHORITY
environment variables are not set. These two variables will be retained
if the org.freedesktop.policykit.exec.allow_gui annotation
on an action is set to a nonempty value; this is discouraged, though, and
should only be used for legacy programs.

REQUIRED AUTHORIZATIONS

By default,
the org.freedesktop.policykit.exec
authorization is required unless an action definition file is
present for the program in question. To require another
authorization, it can be specified using the org.freedesktop.policykit.exec.path annotation on an action (See the section called “EXAMPLE” for details).

EXAMPLE

To specify what kind of authorization is needed to execute the
program /usr/bin/pk-example-frobnicate as
another user, simply write an action definition file like this

and drop it in the
/usr/share/polkit-1/actions directory under
a suitable name (e.g. matching the namespace of the action).
Note that in addition to specifying the program, the
authentication message, description, icon and defaults can be
specified. Note that occurences of the strings
$(user), $(program) and
$(command_line) in the message will be
replaced with respectively the user (of the form "Real Name
(username)" or just "username" if there is no real name for the
username), the binary to execute (a fully-qualified path,
e.g. "/usr/bin/pk-example-frobnicate") and
the command-line, e.g. "pk-example-frobnicate foo
bar". For example, for the action defined above, the
following authentication dialog will be shown:

Note that pkexec does no validation of
the ARGUMENTS passed
to PROGRAM. In the normal case (where
administrator authentication is required every
time pkexec is used), this is not a problem
since if the user is an administrator he might as well just
run pkexec bash to get root.

However, if an action is used for which the user can retain
authorization (or if the user is implicitly authorized), such as
with pk-example-frobnicate above, this
could be a security hole. Therefore, as a rule of thumb,
programs for which the default required authorization is
changed, should never implicitly trust user input (e.g. like any
other well-written suid program).